Method for passing a fluid through a moving bed of particles

ABSTRACT

A process for contacting a bed of particulate material, usually catalyst, with a transverse flow of fluid is disclosed. The particulate material moves or is prevented from not moving, while the fluid passes through the bed at a rate greater than the stagnant bed pinning flow rate. This invention is applicable to hydrocarbon conversion processes and allows for higher fluid throughput rates compared to prior art processes.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/062,689, filed Jan. 31, 2002 now U.S. Pat. No. 6,814,857, theteachings of which are hereby incorporated in their entirety herein byreference.

FIELD OF THE INVENTION

This invention relates generally to the field of fluid-particle contactand more specifically to a method for operation of the moving beds ofradial or horizontal flow fluid-solid contacting devices. Morespecifically, this invention is related to a method for the contactingof a hot fluid stream with particulate material in a particle bed fromwhich particles are continuously or periodically added and withdrawn.

BACKGROUND OF THE INVENTION

A wide variety of processes use radial or horizontal flow reactors toeffect the contact of a compact bed of particulate matter with a fluidand in particular a gaseous stream. These processes include hydrocarbonconversion, adsorption, and exhaust or flue gas treatment. In most ofthese processes, contact of the particulate material with the fluiddecreases the effectiveness of the particulate material in accomplishingits attendant function. In order to maintain the effectiveness of theprocess, systems have been developed whereby particulate material issemi-continuously withdrawn from the contacting zone and replaced byfresh particulate material so that the horizontal flow of fluid materialwill constantly contact a compact bed of particulate material having arequired degree of effectiveness. A moving bed system has the advantageof maintaining production while the catalyst is removed or replaced.Typical examples and arrangements for such systems can be found in U.S.Pat. Nos. 3,647,680; 3,692,496; and 3,706,536; the contents of each ofwhich are hereby incorporated by reference. A good example of the way inwhich moving bed apparatus has been used for the contacting of fluidsand solids is found in the field of petroleum and petrochemicalprocesses especially in the field of hydrocarbon conversion reactions.Many hydrocarbon conversion processes can also be effected with a systemfor continuously moving catalyst particles as a compact column undergravity flow through one or more reactors having a horizontal flow ofreactants. One such process is the dehydrogenation of paraffins as shownin U.S. Pat. No. 3,978,150, and another such process is thedehydrocyclodimerization of aliphatic hydrocarbons.

Another well-known hydrocarbon conversion process that uses a radialflow bed for the contact of solid catalyst particles with a vapor phasereactant stream is found in the reforming of naphtha boilinghydrocarbons. This process uses one or more reactors. Typically, thecatalyst particles enter the top of a first reactor, flow downwardly asa compact column under gravity flow, and are transported out of thefirst reactor. In many cases, a second reactor is located eitherunderneath or next to the first reactor. Catalyst particles again movethrough the second reactor as a compact column under gravity flow. Afterpassing through the second reactor, the catalyst particles may passthrough additional reactors before collection and transportation to aregeneration vessel for the restoration of the catalyst particles by theremoval of coke and other hydrocarbon by-products that accumulate on thecatalyst in the reaction zone.

In the reforming of hydrocarbons using the moving bed system, thereactants typically flow serially through the reactors. The reformingreaction is typically endothermic so the reactant stream is heatedbefore each reactor to supply the necessary heat for the reaction. Thereactants flow through each reactor in a substantially horizontaldirection through the bed of catalyst. The catalyst particles in eachreactor are typically retained between an inlet screen and an outletscreen that together form a vertical bed and allow the passage of vaporthrough the bed. In most cases the catalyst bed is arranged in anannular form so that the reactants flow radially through the catalystbed.

Experience has shown that the horizontal flow of reactants through thebed of catalyst can interfere with the gravity flow removal of catalystparticles. This phenomenon is usually referred to as hang-up or pinningand it imposes a constraint on the design and operation of reactors witha horizontal flow of reactants. Catalyst pinning occurs when thefrictional forces between catalyst particles and the outlet screen thatresist the downward movement of the catalyst particles are greater thanthe gravitational forces acting to pull the catalyst particles downward.The frictional forces occur when the horizontal flow of vapor passesthrough the catalyst bed and the outlet screen. When pinning occurs, ittraps catalyst particles against the outlet screen of the reactor bedand prevents the downward movement of the pinned catalyst particles. Ina simple straight reactor bed, or an annular bed with an inward radialflow of vapors, pinning progresses from the face of the outlet screenand, as the vapor flow through the reactor bed increases, it proceedsout to the outer surface of the bed at which point the bed is describedas being 100% pinned. Pinning between the outlet screen and the outersurface occurs when the frictional forces between catalyst particlesthat resist the downward movement of the catalyst particles are greaterthan the gravitational forces acting to pull the catalyst particlesdownward, thereby trapping catalyst particles against pinned catalystparticles. Once pinning has progressed to the outermost portion of thecatalyst bed, a second phenomenon called void blowing begins. Voidblowing describes the movement of the catalyst bed away from an inletscreen by the forces from the horizontal flow of vapor and the creationof a void between the inlet screen and an outer catalyst boundary. Theexistence of this void can allow catalyst particles to blow around orchurn and create catalyst fines. Void blowing can also occur in anannular catalyst bed when vapor flows radially outward through the bed.With radially outward flow, void blowing occurs when the horizontal flowof vapor creates a void between the inner screen and the inner catalystboundary. Therefore, high vapor flow can cause void blowing in any typeof radial or horizontal flow bed.

The trapping of catalyst particles within a reactor bed that is designedto move continuously causes some catalyst particles to remain in the bedfor a longer time than other catalyst particles that still move freelythrough the bed. As the trapped catalyst particles deactivate andthereby become less effective at promoting the desired catalyticreactions, the reactor bed as a whole exhibits a performance decline,which imposes a direct loss in the production of the desired product. Inaddition, the production of fines can pose a number of problems in acontinuous moving bed design. The presence of catalyst fines increasesthe pressure drop across the catalyst bed thereby further contributingto the pinning and void blowing problems, can lead to plugging in finescreen surfaces, contributes to greater erosion of the processequipment, and in the case of expensive catalysts imposes a directcatalyst cost on the operation of the system. Further discussion ofcatalyst fines and the problems imposed thereby can be found in U.S.Pat. No. 3,825,116, which also describes an apparatus and method forfines removal.

Where possible, horizontal or radial flow reactors are designed andoperated to avoid process conditions that will lead to pinning and voidblowing. This is true in the case of moving bed and non-moving beddesigns. Apparatus and methods of operation for avoiding or overcomingpinning and void blowing problems are shown in U.S. Pat. Nos. 4,135,886;4,141,690; 4,250,018; and 4,567,023, the contents of each of which areincorporated herein by reference. To avoid process conditions that leadto pinning, it has been the practice for many years to operate reactorsof continuous and semi-continuous moving bed designs by maintaining theflow of vapor through the bed of catalyst at a rate that is below therate that will pin catalyst when the bed is stagnant. This rate isreferred to herein as the stagnant bed pinning flow rate.

As explained in further detail in the detailed description below, thestagnant bed pinning flow rate is the fluid rate that prevents at leasta portion of the particles in a bed, which is initially stagnant, frommoving downward when particles are withdrawn from the bottom of the bed.In the past, the stagnant bed pinning flow rate has been estimated usinga theoretical analysis of the mechanics within the stagnant bed ofparticles. A suitable analysis is described in the article written by J.C. Ginestra et al. at pp. 121–124 in Ind. Eng. Chem. Fundam. 1985, 24.The inputs to this analysis are the physical properties of theparticles; the condition of the particle bed; the geometry of theparticle bed and of the screens and walls retaining the bed; thephysical properties of the screens and walls, if any, retaining the bed;the physical properties of the fluid; and the operating conditions ofthe bed. The condition of the particle bed takes into account the solidfraction of the particle bed, the particle-screen static frictionfactor, and the particle-particle static friction factor. Confirmationof this estimate of the stagnant bed pinning flow rate can be obtainedby experiment. The experimental apparatus is a vertically-extended bedof particles between an inlet screen and an outlet screen, with an inletat the top of the bed and an outlet at the bottom of the bed fordownward flow of particles. The apparatus also has a fluid inlet and afluid outlet for cross-flow of a fluid. While the particle outlet isclosed, the particle bed is formed by introducing particles through theparticle inlet in the same manner as particles are introduced throughthe inlet when particles are flowing downward through the bed. Only ashort time after loading in order to ensure that the solid fraction ofthe particle bed is essentially the same as when the particles areloaded, the fluid flow rate through the bed of particles is started at arelatively high rate such that, once downward flow of particles begins,a substantial portion (i.e., about 25–50%) of the particles within thebed is pinned. Then, the particle inlet and outlet are opened, so thatparticles flow downward through the bed. Next, the flow rate is reducedstepwise, with each reduction in flow unpinning some of the particlesthat had been pinned, until the final downward step in flow rate resultsin no pinning of any of the particles. The stagnant bed pinning flowrate can be determined by averaging the penultimate and final flowrates. The precision of the measurement of the stagnant bed pinning flowrate can be improved by decreasing the step size between the penultimateand final flow rates.

Many moving bed design reactors in commercial plants around the worldhave operated for years and even decades at vapor rates below thestagnant bed pinning flow rate described in the preceding paragraph andthereby have successfully avoided any pinning problems. Despitethousands of successful plant-years of operation, catalyst pinning,although rare, can occasionally occur in a radial flow reactor of thecontinuous or semi-continuous moving bed design. When pinning doesoccur, a short procedure is typically used to “unpin” any pinnedcatalyst. First, the vapor flow rate is decreased significantly below,i.e., typically at least 10–50% below, the stagnant bed pinning flowrate, and then the flow rate is increased to a rate which is less thanthe stagnant bed pinning flow rate. The catalyst withdrawal rate may bestopped or decreased, either before, simultaneously with, or after thereduction in vapor rate. If the catalyst withdrawal is stopped ordecreased, then it is usually restarted or increased prior to increasingthe vapor flow rate. In cases of severe pinning where this shortprocedure is unsuccessful, the vapor flow is stopped and the pinnedcatalyst is manually removed from the bed.

Occasionally, commercial reactor beds that are designed to movecontinuously stop moving and come temporarily to rest. This happensintentionally when the reactor bed is designed for semi-continuouscatalyst withdrawal. Depending on the design and operation of thecommercial plant, these periods of time at rest can be in the range offrom as low as 1–2 minutes to as high as 6–12 months, but they arecommonly in the range between 10 minutes and 1 hour. When catalyst flowis resumed, catalyst in these reactor beds does not become pinned, asevidenced by the absence of any symptoms of pinning.

Methods of operation for increasing the vapor flow rate in moving bedprocesses while avoiding pinning problems are sought.

BRIEF SUMMARY OF THE INVENTION

In a surprising discovery, it has now been recognized for the first timethat, when a fluid flows transversely through a moving particle bed andis recovered from a perforated outlet partition, the duration of timeduring which the particle bed has been at rest can significantly affectwhether or not particles pin against the outlet partition. Although thiseffect has been observed in beds containing cohesionless particles, thisdiscovery is believed to be also applicable to particles that exhibitsome cohesion. Exploiting this unexpected discovery, this invention is aprocess for passing a fluid through a bed of particles at a rate greaterthan the previously-defined stagnant bed pinning flow rate, which is theupper limit on fluid flow rate in the prior art processes. By keepingthe particles moving or by decreasing, or keeping at a minimum, theirtime at rest, this invention permits fluid to pass through the particlebed at flow rates far above the stagnant bed pinning flow rate. If theparticles do come to rest while the fluid flow rate exceeds the stagnantbed pinning flow rate, then according to this invention the duration ofthe time at rest is limited either to a specified period of time or to aperiod of time during which the solid fraction increases by only aspecified amount. Of the particles in the bed, those that should move orshould be prevented from not moving in accord with this invention arethose particles that are closest to the outlet screen, since pinninggenerally progresses from the outlet screen. However, a process in whichall of the particles in the bed keep moving is within the scope of thisinvention. In any event, by allowing particle beds to operate at fluidthroughputs outside the constraints imposed by the prior art, thisinvention significantly increases the efficiency and profitability ofprocesses that use new and existing particle beds.

The present discoveries show that, for a given particulate materialmaintained in a bed of a given geometry, the pinning flow rate when thebed is moving (i.e., the moving bed pinning flow rate) is significantlygreater than the stagnant bed pinning flow rate. As explained in furtherdetail in the detailed description below, the moving bed pinning flowrate is the fluid rate that prevents some of the particles in a bed,which is initially moving, from moving downward when particles arewithdrawn from the bottom of the bed. Experimental results show that themoving bed pinning flow rate can be up to 60% higher than stagnant bedpinning flow rates, depending on the bed configuration, particulatematerial, and process conditions. This unexpected discovery led to thepresent invention, which is a dramatic breakthrough process in the sensethat this invention shatters preexisting notions about processingbarriers. This invention surpasses hydraulic constraints that hadpreviously been considered insurmountable. In contrast to the prior artwhich viewed the stagnant bed pinning flow rate as a limit on the flowrate of reactants through a bed of particulate material, this inventionallows for flow rates of reactants through the bed that exceed thestagnant bed pinning rate and which approach the moving bed pinning flowrate. In theory, by using this invention, the flow rate of reactantsthrough the bed can be increased by a factor equal to the ratio of themoving bed pinning flow rate to the stagnant bed pinning flow rate. Inpractice, the reactant flow rate will be increased by less than thatfactor, and will typically increase in the range of about 15% to about60% of the reactant flow rate in the prior art processes. Neverthelessthe economic and technical benefits that accrue from even a 1% increasein the flow rate of reactants through an existing particular bed areenormous.

Without being bound by any particular theory, it is believed that theexplanation why the fluid flow rate can be increased while avoidingpinning problems is that the particle-screen friction is significantlyless when the particulate material is moving relative to the screen thanwhen the particulate material is stagnant relative to the screen, andtherefore pinning decreases. The particle-screen friction is caused byparticles contacting the screen and is expressed by the particle-screenfriction factor. The particle-screen kinematic friction factor expressesthe friction caused by particles moving or sliding against the screen,whereas the particle-screen static friction factor expresses thefriction between particles and the screen when the particles and thescreen are not moving relative to each other. The particle-screenfriction is significantly lower not only because the solid fraction inthe bed decreases but also because the particle-screen kinematicfriction factor itself decreases. In addition to the belief that theparticle-screen kinematic friction factor decreases relative to theparticle-screen static friction factor, it is also believed that theparticle-screen kinematic friction factor is only very weakly dependenton the flow rate of the particulate material. So, as long as theparticulate material is moving or sliding to any extent along thescreen, it is believed that the particle-screen kinematic frictionfactor remains significantly less than the particle-screen staticfriction factor. Thus, the critical characteristic of moving or slidingparticles along the screen in this invention is not how fast they aremoving, but that they are in fact moving.

Similarly, it is also believed, without being bound by any particulartheory, that the particle-particle kinematic friction factor issignificantly less than the particle-particle static friction factor,and also that the particle-particle kinematic friction factor is onlyvery weakly dependent on the flow rate of the particulate materialacross the screen. The particle-particle static friction factorexpresses the friction between solid particles that are not movingagainst each other, and the particle-particle kinematic friction factorexpresses the friction caused by particles flowing against each other.While the particle-screen friction is an important factor in the onsetof pinning at the outlet screen, the particle-particle friction is animportant factor in determining the extent and shape of the volume ofpinned particles in the bed.

It has also now been recognized, as a result of this invention, that formany new moving-bed commercial processes, a semi-continuous moving beddesign in which the bed is at rest for long periods of time may not beoptimum from a process economics viewpoint. In most cases, the capitalthat would be invested in equipment and control systems for asemi-continuous moving bed design would yield a much greater return ifinvested instead on the equipment and systems for a continuous movingbed design, because the continuous moving bed design allows for asignificant increase in processing throughput through an existingparticle bed process, which leads to greater profitability. Investingcapital in a revamp of an existing semi-continuous moving bed processesinto a continuous process will also yield a high return on investment.

In addition, this invention provides a method for unpinning particles ina particle bed. First, and optionally, the addition and withdrawal ofparticles to and from the bed is stopped. Then, the fluid flow rate isdecreased to a rate that is less than the stagnant bed pinning flow rateof the bed. Finally, if the particle flow to and from the bed wasstopped, then the particle flow is resumed.

This invention also provides a method of raising the fluid flow ratethrough the bed to a rate that is more than the stagnant bed pinningflow rate. Initially, the fluid flow rate is less than the stagnant bedpinning flow rate, and then the addition and withdrawal of particles toand from the bed is begun. Finally, the fluid flow rate is increased toa rate above the stagnant bed pinning flow rate.

This invention stands in stark contrast to prior art fluid-particlecontacting processes. Embodiments of this invention that keep theparticles moving differ from two prior art processes, namely thoseprocesses where the particles move only when the fluid rate is less thanthe stagnant bed pinning flow rate and those processes where theparticles are not moving when the fluid rate exceeds the stagnant bedpinning flow rate. Embodiments of this invention that regulate the timeduring which the particles are not moving when the fluid rate is greaterthan the stagnant bed pinning flow rate differ from the prior artprocess in which the particles do not move at all while the fluid rateis greater than the stagnant bed pinning flow rate. Such embodiments arealso different from the prior art process in which the particles move attimes and do not move at other times, but the fluid rate is less thanthe stagnant bed pinning flow rate.

In a broad embodiment, this invention is a process for passing a fluidthrough a bed of particulate material. A particulate material ismaintained in a vertically extended bed having a fluid inlet face. Thebed is maintained between the fluid inlet face and an outlet partitionthat has a perforated section extending over at least part of itslength. The size of the perforations retains the particulate materialwhile permitting fluid flow through the perforations. The particulatematerial is withdrawn from the bottom of the bed. An inlet fluid passesto the fluid inlet face and transversely through the bed. An outletfluid is recovered from the perforated section of the outlet partitionat an operating flow rate that is not less than a stagnant bed pinningflow rate. In another embodiment, the particulate material in the bed isprevented from coming to rest for a period of time.

Other objects, embodiments and details of this invention are disclosedin the following detailed description.

INFORMATION DISCLOSURE

The article written by J. W. Carson et al. at pp. 78–90 in ChemicalEngineering, April 1994, which is incorporated herein by reference andhereinafter referred to as “Carson,” teaches that, in the field of bulksolids handling in bins, hoppers, and feeders, the time that a bulksolid is stored at rest can affect the flow behavior of the bulk solid,since time of storage at rest can determine in part the cohesiveness andthe frictional properties of a bulk solid. Carson teaches that, when amaterial resides in a bin or hopper for a period without moving, it canbecome more cohesive and difficult to handle, and that such cohesion maybe caused by settling and compaction, crystallization, chemicalreactions, and adhesive bonding.

The article written by J. Marinelli et al. at pp. 22–28 in ChemicalEngineering Progress, May 1992, which is incorporated herein byreference and hereinafter referred to as “Marinelli,” teaches that thetime that a bulk material is at rest can influence the wall friction andcan affect the determination of the size of the outlet of a bin orhopper.

The article written by J. C. Ginestra et al. at pp. 121–124 in Ind. Eng.Chem. Fundam. 1985, 24, analyzes the mechanics of pinning of a bed ofparticles in a vertical channel by a cross flow of gas.

The paper written by S. B. Savage, which is published at pp. 261–282, inthe book entitled Mechanics of Granular Materials—New Models andConstitutive Relations, edited by J. T. Jenkins et al., Studies inApplied Mechanics 7, Elsevier Science Publishing Co., Inc. New York,1982, reviews and discusses models that have been applied to predict theflows of dry, cohesionless granular materials down inclined surfaces.

The paper written by H. J. Jaeger et al. at pp. 1523–1531 in Science,Vol. 255, Mar. 20, 1992, describes inhomogeneity, maximum and minimumpacking densities, and stress-carrying networks in particle beds.

The article entitled, “Hidden in the Hopper: A Secret of Physics,”written by James Glanz and published in The New York Times, Jan. 9,2001, section F, page 3, column 1, describes the difficulties thatresearch physicists have had in devising a theory of “jamming” ofgranular materials in hoppers that takes into account the “nastyrealities” of industrial practice, such as the effects of frictionbetween particles of the granular material, humidity, other atmosphericconditions, and three-dimensional flow patterns.

DETAILED DESCRIPTION OF THE INVENTION

The fluid-particle contacting process of this invention can be appliedto any type and form of bulk solid. A bulk solid is a particulatematerial comprising particles. Bulk solids include a large class ofmaterials, including coal, ores, minerals, chemicals, catalysts,cereals, grains, plastics, which exist in a large number of differentparticle forms, including powders, granules, flakes, chips, crystals,and agglomerates, and in a large number of particle shapes, includinground, angular, spherical, cylindrical, and irregular. See, for example,Table 21–4 on p. 21–6 in Perry's Chemical Engineers' Handbook, 7^(th)Edition, edited by R. H. Perry et al., McGraw-Hill Book Co., New York,N.Y., USA, 1997, hereinafter referred to as “Perry.” The particles maybe formed either by size reduction or enlargement processes, such asthose described generally in Sections 17 and 20 in Perry. Particlescontaining, for example, alumina or silica can be formed by sprayprocesses such as spray drying, which produces particles of irregularshapes in a distribution of sizes. Spheres of gamma alumina may beproduced by the well-known oil drop method, which is described in U.S.Pat. No. 2,620,314.

The particles preferably have good flowability characteristics. A personof ordinary skill in the art of conveying bulk solids can selectparticles having properties which are known to help ensure smoothparticle flow. For example, Carson teaches parameters of bulk solidswhich are useful for predicting flow behavior of bulk solids and whichare believed to be relevant to the suitability of particles for use inthis invention. These parameters include cohesive strength, frictionalproperties, sliding at impact points, compressibility, permeability,segregation tendency, friability, abrasiveness, and pneumatic-conveyingcharacteristics.

The cohesive strength of the particles is preferably minimized, and morepreferably the particles are cohesionless, because the fluid rate thatpins particles decreases as the cohesive strength of the particlesincreases. As used herein, particles are “cohesionless” if, when twoparticles are placed together and then separated by force, the forcerequired for separation is zero Newton. Many commercial catalysts usedin commercial moving bed processes have negligible attractive forcebetween particles and may be considered “cohesionless.” However, it isbelieved that particles that exhibit some cohesive strength may also besuitable for use in this invention. Carson teaches that the cohesivestrength is a function of several characteristics, including moisture,particle size and shape, temperature, time of storage at rest, andchemical additives. In addition, it is believed that particles whichhave a relatively high Young's modulus (i.e., which exhibit less strain)under compressive stress tend to have lower cohesive strength.Preferably the sizes of the particles in the particulate material areuniform, but where the sizes of the particles are not uniform, then thesize distribution may affect cohesive strength, since the particles maysegregate or smaller particles may fill voids between larger particles.Particles that have an average size of less than half of the averagesize of all the particles comprise generally less than 1 wt-%,preferably less than 0.1 wt-%, more preferably less than 0.01 wt-%, evenmore preferably less than 0.001 wt-%, and still more preferably lessthan 0.0001 wt-%, of all the particles. Finally, it is believed that thedensity of the particles, if increased by consolidation pressure, canalso increase cohesive strength.

The particles may be catalysts in a reaction zone in which a reactantfluid contacts the catalytic particles. Solid catalysts are widely used,and most solid catalysts are used as porous particles in a bed ofparticles. Suitable solid catalysts may include acids, bases, metaloxides, metal sulfides, metal hydrides, metals, alloys, andtransition-metal organometallic catalysts. Solid catalysts are selectedprimarily for their activity, selectivity, and stability with respect toa particular reaction. Perry describes catalysis by solids at pages23–26 to 23–29 and pp. 23–36 to 23–38, the teachings of which areincorporated herein by reference.

The fluid-particle contacting process of this invention can be appliedto any form of moving bed wherein the particulate material flowsdownward in a compact bed and fluid contacts the particles by transversemovement through the compact bed. The terms “compact bed” or “compactcolumn” of particulate material refer to a state wherein the particlesrest on top of each other whether moving or stationary—as opposed to afluidized bed where gas flow creates lift on the particles to createvoid spaces between the particles in a fluidized movement of particles.

The method of this invention is particularly adapted for the radial orcross flow of a fluid through the bed of particles. The bed of particlesis typically maintained as a relatively thin and vertically extendedlayer of particles through which the fluid passes transversely. Thethickness of the bed over its height may also vary and is advantageouslyin some regeneration processes arranged to increase down the length ofthe bed. The particle pathways in the bed have a vertical downwardcomponent. Although the particle pathways may have a horizontalcomponent, preferably they do not. The fluid streamlines in the bed havea horizontal component and may have a vertical component, and if presentthe vertical component may be upward but is preferably downward. If theparticle pathways have a horizontal component, that component is usuallyin the direction of the horizontal component of the fluid streamlines.

The bed of particles, in its most general arrangement, has a fluid inletface through which fluid enters the bed, and the bed is maintainedbetween the fluid inlet face and an outlet perforated partition. Thefluid entering the bed can be distributed across the fluid inlet face ofthe bed by an inlet perforated partition, by the fluid inlet face and asuitable depth of particles, or by both. Without an inlet perforatedpartition, the shape of the fluid inlet face of the bed of particles isdetermined by factors including gravity, the angle of repose of theparticles, how the bed was formed, the flowability of the particles,operating conditions, and the composition and flow rate of the fluid.Although the fluid inlet face is preferably vertically extended, thefluid inlet face may be horizontally extended. An embodiment of thisinvention using a horizontally-extended fluid inlet face comprisesintroducing both fluid and particles to the top of the verticallyextended bed, passing the particles downward through the bed andwithdrawing them from the bottom of the bed, and withdrawing fluid fromthe side of the bed. In this embodiment, the flow direction of fluidchanges within the bed from vertically downward to horizontal, and thuswithin the bed the fluid passes transversely at least at some pointwithin the bed.

In embodiments of this invention where particles are used to distributethe fluid entering the bed, such as those without an inlet perforatedpartition, the single bed can function as if there were two sub-bedsthrough which fluid passes in series. In the first sub-bed, the fluid isdistributed, and in the second sub-bed the fluid contacts the particles.In this embodiment, the fluid inlet face is that of the second sub-bed,which is within the single bed.

The surface of the bed of particles may have a flat shape, as in thecase of a bed having the form of, for example, a cube, a rectangularparallelepiped, a prism, or a frustum of a pyramid. But in most casesthe surface of the bed is curved and the bed itself is arranged in anannular form by inner and outer perforated partition elements. Apreferred embodiment of this invention is a process for contacting areactant fluid with catalytic particles in a reaction zone which usesscreens to contain the catalyst particles in a configuration while thereactant fluid passes radially—either inwardly or outwardly—through thebed. The terms “inlet screen” and “outlet screen” are used generally todescribe any type of perforated element that may distribute or collectthe fluid while containing, or retaining, the catalyst particles in aconfined space. Suitable screen elements consisting of profile wire orother perforated members are well known to those skilled in the art.Another preferred form of distributing the fluid and providing acontainment space is using extended conduits that have a scallopedshaped profile and are commonly referred to as “scallops”.

The construction, material, and surface condition of each screen,especially of the outlet screen, preferably minimize friction betweenparticles and the screen. A person of ordinary skill in the art ofscreen technology can select a screen that minimizes this friction andwhose friction will not increase during service due to corrosion orroughening or abrasive wear. Marinelli teaches factors which influencefriction between a bulk solid and the wall of a hopper or bin and whichare believed to be relevant to the suitability of screens for use inthis invention. These parameters include cohesive strength, frictionalproperties, sliding at impact points, compressibility, permeability,segregation tendency, friability, abrasiveness, and pneumatic-conveyingcharacteristics. The angle between each screen surface and thehorizontal may be from 45 to 135°, but is preferably 90°. The angle thatone screen surface makes with the horizontal may be slightly more orless than that of another screen surface in the case of a tapered bedwhose thickness varies over its length.

The arrangement of the bed of particles, the inlet perforated partition,and the outlet perforated partition preferably ensures uniform flow ofthe fluid through the bed of particles. The flux of fluid (i.e., thequotient of the fluid flow rate through a given area of the face of theoutlet screen divided by the face area of the outlet screen) through anysquare foot of face area of the outlet screen is generally within 5%,and preferably within 1%, of the average flux of all the fluid flowingthrough the entire face area of the outlet screen. A preferredarrangement of the bed and the partitions is disclosed generally in theU.S. Pat. No. 4,567,023, and particularly at column 8, line 46 to column9, line 40. Accordingly, the velocity head of the fluid stream in aninlet manifold defined in part by the inner screen and the velocity headof the fluid stream in an outlet manifold defined in part by the outerscreen are balanced, as described in U.S. Pat. No. 4,567,023.Preferably, the velocity head of the fluid stream at the lower terminalend of the inlet manifold is 0, and the velocity head of the fluidstream at the lower terminal portion of the outlet manifold issubstantially 0. Another arrangement of the bed and partitions isdescribed in U.S. Pat. No. 3,706,536.

The bed of particles has a solid fraction, which is the volumetricfraction of the total bed volume that is occupied by the solidparticles. Solid fraction, as used herein, equals one minus the voidfraction. Void fraction refers to the volume fraction of the bed ofparticles that is occupied by the void spaces between the particles andexcludes volume within the pores of the particles. The recognition thatsolid fraction is an important parameter in determining whether or notparticles in a bed pin is one of the important discoveries that led tothis invention. Unexpectedly, it has been discovered that as the solidfraction increases, the tendency of the bed to pin at a given fluid rateincreases. For particles having a uniform size, the solid fraction for amoving bed is generally from about 0.57 to about 0.72, and moretypically from about 0.60 to about 0.65. As a general rule, the solidfraction for a stagnant bed is greater than that for the same bed whenits particles are moving. At a given point in a particle bed, thedecrease in solid fraction that occurs when a stagnant bed starts movingis usually relatively large compared to any further decreases that occurwhen the downward particle velocity increases in an already-moving bed.

The void spaces between particles, even in a compact bed wherein theparticles rest on top of each other, can increase or decrease in size.Generally, the solid fraction of a bed depends on several factors,including the manner in which the particle bed was formed, the velocitydistribution of the particles, the size distribution of the particles,whether or not the bed has been or is being vibrated, and whether or nota fluid passes through the bed of particles. If a fluid passes throughthe bed of particles, the composition, flow rate, and operatingconditions of that fluid are also factors. In stagnant compact particlebeds, another factor is the time at rest for the nonmoving particles,because the solid fraction increases as the time that the particles areat rest increases. Since these factors may be different in differentregions of a particle bed, the solid fraction in the bed may not beidentical in all regions of the particle bed. For example, in avertically-extended particle bed, the solid fraction at a given verticalelevation may vary over the cross-section of the bed. Also, the solidfraction may be different at different vertical heights of the bed.Variation in the solid fraction of a moving bed with height can occur ifthe cross-sectional area of the bed varies with height, since the solidfraction will be relatively high when the cross-sectional area isrelatively high and the particle velocity is relatively low, even if theparticle flow rate is the same at each elevation throughout the bed.

The solid fraction of a bed can be determined in three ways. First, thesolid fraction can be computed by dividing the bed density by theparticle density. The bed density is the density of the particle bed andequals the weight of particles in the bed divided by the volume of thebed, and the particle density is the weight of a particle divided by thevolume of the particle. Second, for at least some particle beds, thesolid fraction of a particle bed can be determined from measurements ofthe electrical capacitance of the particle bed using one or morecapacitance probes that are immersed in the particle bed.

A third way of determining the solid fraction of a particle bed is bypressure drop. If a fluid passes through the bed of particles, the solidfraction in a bed can be determined by measuring the pressure drop alonga fluid streamline across or in the bed of particles and then computingthe solid fraction of the bed of particles using the Ergun equation,which relates the pressure drop to the solid fraction. See Equation6–166 on p. 6–38 in Perry. Assuming turbulent fluid flow and that allother conditions within the bed are constant, then the following formulais an adequate approximation of the relationship between pressure drop(ΔP), the fluid superficial velocity (V), and the solid fraction (e):ΔP=(K)(V ²)(e)/(1−e)³,where K is a constant. Based on this formula, it is apparent that, at aconstant superficial velocity, an increase in e from 0.56 to 0.58increases ΔP by 19%, a change which can be easily measured byconventional differential pressure instrumentation. Therefore,monitoring the pressure drop across a bed of particles at a constantsuperficial velocity permits detecting even small changes in solidfraction within the bed of particles. However, it should be pointed outthat this formula determines an average solid fraction for the bed ofparticles as a whole, but in fact the solid fraction may not be uniformthroughout the bed, and some regions within the bed may have solidfractions which are higher or lower than the average solid fraction.

To determine the solid fraction of a stagnant bed of particles using theformula in the preceding paragraph, pressure drops are measured when thebed is stagnant for at least two, and preferably about ten, differentsuperficial velocities. From these data, the value of K can bedetermined by averaging or by a least-squares analysis, and then thevalue of e for the stagnant bed can be determined. To determine thesolid fraction of a moving bed of particles, the pressure drop ismeasured when the bed is moving for at least one superficial velocity,and preferably for about five superficial velocities. Except for thefacts that the superficial velocities may be different and that the bedis moving, all other operating and measuring conditions of the bed arethe same as during the determination of the solid fraction of thestagnant bed. Thus, the value of K determined from the measurementswhile the bed was stagnant is directly applicable to the measurementsmade while the bed is moving, so that the solid fraction for the movingbed can be determined from the pressure drop(s) obtained from the movingbed measurements. From this description of the use of the above formula,it should be apparent to a person of ordinary skill in the art that thevalue of K can also be determined using measurements of a moving bedrather than those of a stagnant bed.

It is important in the present invention that, when the fluid rateexceeds the stagnant bed pinning flow rate, that the particles are keptmoving or that the time during which the particles are at rest isregulated within prescribed limits. The stagnant bed pinning flow rateis defined as the fluid rate that prevents at least a portion of theparticles in a bed, which is initially stagnant, from moving downwardwhen particles are withdrawn from the bottom of the bed. It is assumedthat the configuration of the particle bed is such that, when no fluidpasses through the bed, at least a portion of the particles in the bedmay move downward within the bed when particles are withdrawn from thebottom of the bed by opening the particle outlet. The flow rate of thefluid is in excess of the stagnant bed pinning flow rate when at least aportion of the particles that were able to move downward when no fluidpassed through the bed cannot move downward when the fluid is passingthrough the bed. That is, but for the flow rate of the fluid being inexcess of the stagnant bed pinning flow rate, the particles in the bedthat could move downward when the bed was initially stagnant and theparticle outlet was opened are unable to move downward., despite thefact that the particle outlet is opened.

As used herein, keeping the particles moving means that the particlesare always moving in a downward direction, even if the velocity ofmovement in the downward direction is relatively small. Movement ofparticles in the downward direction in one portion of the bed cangenerally be achieved by withdrawing particles from a lower portion ofthe bed, usually from the bottom of the compact bed, which in turnallows particles higher in the bed to move downward because theparticles in the bed rest on top of each other even when moving. It isrecognized that, even when the particles are not being withdrawn from alower portion of the compact bed, that some movement of the particlesmay occur by forces other than those which are present only whenparticles are being withdrawn from below. Those other forces includevibration, settling, compaction, thermal expansion, or thermalcontraction of the particle bed. It is believed that movement due to anythese phenomena individually or even in combination, in the absence ofwithdrawal of particles from a lower portion of the bed, is insufficientmovement for purposes of this invention.

In embodiments of this invention that keep the particles moving, theminimum downward component of the velocity of the particles for anyspecific fluid-particle contacting process depends on many factors thatare specific to that particular process. In these embodiments, theminimum downward velocity is greater than zero, of course. But, sincethis invention has general applicability to any fluid-solid contactingprocess, it is not practical to list herein all of the factors that canaffect the minimum downward velocity for all possible processes in whichthis invention can be used, or to specify definite minimum velocitiesfor any particular process. For example, and because this invention isgenerally applicable to hydrocarbon conversion processes catalyzed bysolid catalyst particles, the factors for two hydrocarbon conversionprocesses—catalytic dehydrogenation and catalytic reforming—arepresented here. In these processes, the factors include the compositionand properties of hydrocarbon feed; operating conditions includingtemperature, pressure, and space velocity; catalyst physical propertiesincluding size, friction properties, and cohesiveness; nature of thereactions that take place in, on, and outside of the catalyst; shape,dimensions, and configuration of the catalyst bed; residence time ofcatalyst in the catalyst bed; catalyst performance including itsactivity, selectivity to products and byproducts, and its deactivationrate and mechanism; and the particle handling capacities of theequipment that supplies particles to the bed and of that which withdrawsparticles from the bed. However, a person of ordinary skill in the artcan determine a suitable minimum velocity of particles for any givenparticle bed by monitoring the operating conditions of the particle bedin response to different particle velocities. It is believed that theminimum downward component of the particle velocity is generally greaterthan 0.02 mm/min, typically greater than 0.2 mm/min, and more typicallygreater than 2.0 mm/min, and in some instances the particles aremaintained at least at 20 mm/min.

It is also within the scope of this invention to regulate the timeduring which the particles are at rest within prescribed time limits.That is, processes in which at times the particles are not movingdownward are nevertheless within the scope of this invention, providedthat, within a specified period of time the downward movement ofparticles is resumed. As used herein, the term “at rest,” means that thedownward component of the velocity of the particles is zero. Particlesthat are “at rest” are sometimes referred to herein as being “stagnant.”Stopping movement of particles in the downward direction in one portionof the bed can generally be achieved by stopping the withdrawal ofparticles from a lower portion of the bed, such as from the bottom ofthe compact bed, which prevents particles higher in the bed from movingdownward because they rest on particles lower in the bed. As previouslymentioned, some movement of particles in the bed may occur even whenparticles are not being withdrawn from below, due to previouslymentioned phenomena including settling within the bed. Since thevelocity of any such movement is small, particles that move only becauseof one or more of these phenomena—alone or in combination—in the absenceof withdrawal of particles from a lower portion of the bed, have forpurposes of this invention, zero downward velocity.

In embodiments of this invention in which the particles are alternatelyat rest for a period of time and then moving for a period of time, theduration of the at-rest time period must be non-zero, but the maximumduration of this stagnant time period for any specific fluid-particlecontacting process depends on many factors that are specific to theparticular process. These factors include the same list of factorspreviously listed as affecting the minimum downward velocity. For thisreason, it is not possible to specify a definite maximum duration oftime for any particular process, including hydrocarbon conversionprocesses. However, a person of ordinary skill in the art can determinea suitable time period of being at rest for any given particle bed bymonitoring the operating conditions of the particle bed in response tochanges in the time of stoppage of the particle flow. It is believedthat the maximum duration of time that the downward velocity is zero isgenerally less than about 72 hr, typically less than about 24 hr, andmore typically less than about 1 hr. The particles should be preventedfrom being at rest for a period of more than 72 hr generally, 24 hrtypically, and 1 hr more typically. However, for some particles and someprocesses, the maximum time at rest may be very short indeed, and it isexpected that in some processes, this time may be in the range of fromabout 1 sec to about 1 hr, more usually between about 1 min and about 30min, and even more typically between about 5 min and about 20 min.Particles in processes such as these should be prevented from being atrest for a period of more than 1 hr generally, 30 min usually, and 5 minmore typically. Because the solid fraction increases as the time at restincreases, the time at rest should be sufficiently short that the solidfraction in the bed increases by less than 0.04 generally, 0.02preferably, and 0.01 more preferably.

Where the particles are alternately at rest for a period of time andthen moving for a period of time, the magnitude of the downwardcomponent of the average particle velocity is generally greater than0.02 mm/min, typically greater than 0.2 mm/min, and more typicallygreater than 2.0 mm/min, and in some instances the particles aremaintained at least at 20 mm/min.

The particles which keep moving or which are alternately at rest andmoving comprise, in one embodiment of this invention, all of theparticles in the particle bed. In this embodiment, the particles in theparticle bed move downward in a mass-flow pattern, such as thatdescribed in Marinelli for flow in bins and hoppers, rather than in afunnel-flow pattern such as ratholing. Although Marinelli is notdirected to cross-flow of gas through downflowing solids and even thoughMarinelli states that solid/gas interactions are very complex and oftencounterintuitive, it is believed that a person of ordinary skill candesign a particle bed configuration in which vapor flows cross-wise andwhich is designed to ensure that, if some of the particles in the bedmove downward, then most if not all of the other particles in the bedmove downward also. U.S. Pat. No. 4,567,023, for example, describes amultiple stage reactor system in which allows little if any stagnationof catalyst by using an annular-form catalyst collecting zone at thebottom of the particle bed and by removing the reactant stream from thetop of the outlet screen. However, it is not a requirement of thisinvention that all of the particles in the particle bed move downwardwhen particles are withdrawn from the bottom of the particle bed. Someparticles may remain stagnant in the particle bed for reasons other thanpinning, because of factors similar to those described in Marinelli,including, for instance, the steepness and smoothness of the screens andwalls which contain the particle bed. In some embodiments of thisinvention, particles that are relatively far from the outlet screen, maynot move at all when particles are withdrawn from the bed, and theseparticles are sometimes referred to as “heel particles” or, in the caseof catalytic particles, “heel catalyst.” Generally, however, particlesthat are relatively near to the outlet screen move. Particles that moveinclude those that are within a distance from the outlet screen ofgenerally from about 0 to about 2, commonly from about 0 to about 5, andmore commonly from about 0 to about 10, nominal particle diameters.Expressed in terms of a percentage of the distance from the outletscreen to the fluid inlet face of the particle bed, the particles thatmove are generally from about 0 to about 1%, commonly from about 0 toabout 5%, and more commonly from about 0 to about 10%, of the distance.Of course, particles outside of the ranges set forth in the precedingsentences may also move downward.

It is beyond the scope of this invention to describe all of the factorsin addition to the flow rate of the fluid that may affect whether or notparticles at any given distance from the outlet screen move whenparticles are withdrawn from the bottom of the particle bed. Forcompleteness, however, these other factors are believed to include thedistance and orientation relative to the outlet screen of the particleoutlet from the particle bed, as well as the flow rate of particlesthrough the particle outlet. The particle discharge rate is believed tohave little effect on whether or not all the particles in the bed moveat once, unless the discharge rate is extremely high and initiatessignificant particle-particle turbulence in the bed. The particleoutlet, on the other hand, is believed to have a significant effect onwhich particles move within the bed. In general, the shorter thehorizontal distance between the opening in the particle outlet and thesurface of the outlet screen, the more likely it is that particles nearthe outlet screen will move when particles are withdrawn through theparticle outlet. Also, as particles across the entire width of theoutlet screen move downward, their downward direction is preferablypointed directly into the particle outlet through which they exit theparticle bed. In the case of a flat outlet screen, for example, theopening of the particle outlet is preferably horizontally extended andis as long as the outlet screen is wide. In the case of a cylindricaloutlet screen the opening for the particle outlet preferably extendsaround the entire circumference of the outlet screen, such as is shownin U.S. Pat. No. 4,567,023, for example. Similarly, the greater thewidth of the opening of the particle outlet, the more likely it is thatmore particles in the bed will move downward when any of the particlesmove downward. For example, if the entire cross-sectional area of thebed at the elevation of the bottom of the outlet screen emptied into aparticle outlet having the same cross-sectional area as that of the bed,then particles throughout the bed would be expected to move downwardwhenever the particles at the surface of the outlet screen moveddownward. To the extent that the cross-sectional area of the particleoutlet is reduced, areas of stagnant particles in the bed may exist,even if all the particles in the bed along the surface of the outletscreen are moving downward. More than one particle outlet opening may beused.

Generally, particles that are neither pinned nor otherwise stagnant movedownward through the bed at velocities that are determined by processingrequirements of the fluid-particle contacting process. These, in turn,relate to the nature of the fluid and the particles. However, velocitiesare typically in the range of from about 1 to 20 mm/minute, and morecommonly in the range of from about 1 to about 10 mm/minute.

The velocity of the particles can be determined by measuring thedistance that particles move during a given time interval. This can bedone with the human eye, unaided or with the aid of magnification, orwith a camera. Particle velocities as low as about 0.015 mm/min, andeven lower, can be measured readily. The accuracy and precision of thevelocity measurement can be improved by increasing the time interval orby increasing the magnification. Where the particles are alternately atrest for a period of time and then moving for a period of time, thevelocity computed by this method is usually an average based on thedistance moved and the time elapsed over several consecutive periods ofmovement and rest.

The average velocity of particles in the bed can also be estimated fromthe average residence time of particles in the bed. Average particlevelocity in the bed can be determined by dividing the volumetric flowrate of particles through the bed in the downward direction by thehorizontal cross-sectional area of the bed, with consistent length unitsfor volume and area. Average particle residence time in the bed can alsobe determined by marking particles that are to be added to the bed andthen measuring the time that elapses between when the marked particlesenter the bed and when the marked particles exit the bed. Suitablemarking methods depend on the function and operating conditions of thebed and on the nature of the particles themselves. But the markingmethod should not significantly change the flowing properties of themarked particles relative to that of the unmarked particles, in order toensure that the flow of the marked particles through the bed isrepresentative of that of the unmarked particles. If the particlesentering and exiting the bed can be visually observed, then a simplemethod of marking is dyeing the particles a color different from that ofthe other particles. When the entering and exiting particles cannot beseen, radioisotope technology marking, which uses marked particles thatare identical to the unmarked particles, except that they contain aradioisotope and they function as a tracer. Whatever the method ofmarking, the concentration-time profiles of the inlet pulse of markedparticles entering the bed and of the outlet pulse exiting the bed canbe measured using detectors (i.e., visually or radioisotopically), andthe residence time of the particles in the bed can be determined.Further information on residence time studies using radioisotope tracingtechniques can be obtained from Synetix Process Diagnostics, P.O. Box 1,Belasis Hall Technology, Billingham, Cleveland, TS23 1LB, UnitedKingdom.

While the stagnant bed pinning flow rate places a lower limit on thefluid flow rate when practicing this invention, the moving bed pinningflow rate places an upper limit on the fluid flow rate. The moving bedpinning flow rate is defined as the fluid rate that prevents at least aportion of the particles in a bed, which is initially moving, frommoving downward when particles are withdrawn from the bottom of the bed.It is assumed that the configuration of the particle bed is such that,when no fluid passes through the bed and the particle outlet at thebottom of the bed is opened, at least a portion of the particles in thebed move downward and particles are withdrawn from the bottom of thebed. The flow rate of the fluid is in excess of the moving bed pinningflow rate when at least a portion of the particles that were able tomove downward when no fluid passed through the bed cannot move downwardwhen the fluid is passing through the bed. That is, but for the flowrate of the fluid being in excess of the moving bed pinning flow rate,particles in the bed that could otherwise move downward are unable tomove downward.

The moving bed pinning flow rate can be determined in a manner similarto that described previously for the stagnant bed pinning flow rate.Thus, the moving bed pinning flow rate can be estimated using atheoretical analysis of the mechanics within the moving bed of particlesusing the analysis described in the previously-mentioned article writtenby J. C. Ginestra et al. The inputs to this analysis are the same asthose previously listed for the stagnant bed pinning flow rate analysis,except that the “history” of the particle bed includes how the particleswere loaded into the bed; whether or not the particles were stoppedafter loading; when and for how long the particles were stopped, if theywere stopped; and how fast and for how long the particles have moved orhave been moving. The moving bed pinning flow rate can also be measuredby experiment using an apparatus such as that previously described formeasuring the stagnant bed pinning flow rate. After establishing theparticle bed with its initial conditions and its “history,” and whilethe particle inlet and outlet are open, the fluid flow rate through thebed of particles is either started or maintained at a relatively lowrate so that, while downward flow of particles takes place, none of theparticles within the bed are pinned. Next, the flow rate of fluid isincreased stepwise, with each increase in flow rate approaching the flowrate that causes some of the particles to be pinned, until the finalupward step in flow rate results in pinning of some of the particles.The moving bed pinning flow rate can be determined by averaging thepenultimate and final flow rates. The precision of the measurement ofthe moving bed pinning flow rate can be improved by decreasing the stepsize between the penultimate and final flow rates.

When practicing this invention, the fluid flow rate is preferably fromabout 105% to about 160%, and more commonly from about 120% to about140%, of the stagnant bed pinning flow rate. Also, when practicing thisinvention, the fluid flow rate is preferably from about 50% to about95%, and more commonly from about 60% to about 90%, of the moving bedpinning flow rate.

The fluid for use in this invention may be a liquid but is preferably agas. The fluid may be inert with respect to the particles, may exchangeheat or mass with the particles, or may be involved in reactions whichoccur on the particles or in the fluid. Preferably the fluid is a singlephase at the operating conditions of the particle bed. Suitable fluidsinclude inorganic gases such as air, hydrogen, water (i.e., steam),nitrogen, and oxygen; organic gases such as hydrocarbon vapors includingC₁ to C₂₅ hydrocarbons, including paraffins, olefins, naphthenes, andaromatics; and mixtures of organic and inorganic gases.

This invention is applicable to a number of hydrocarbon conversionprocesses in which a particulate material is maintained in a verticallyextended moving bed while gas flows transversely or in cross flowthrough the bed. Examples of these processes include, but are notlimited to dehydrogenation of hydrocarbons to olefins or aromatics,reforming of naphtha, and dehydrocyclodimerization of liquid paraffingases to naphthenes or aromatics. This invention is also applicable toprocesses for the regeneration or reactivation of deactivatedhydrocarbon conversion catalysts. The conditions for carrying out theseprocesses are well known in the art and are presented here forcompleteness.

In the case of a moving bed reaction zone, fresh catalyst particles arefed to the reaction zone, which may be comprised of several reactors,and the particles flow through each zone by gravity. The feed stream isusually preheated by any suitable heating means to the desired reactiontemperature and then passed into the reaction zone containing a bed ofcatalyst. The reactants may be contacted with the catalyst bed in eitherupward, downward, or radial flow fashion. Generally, the reactants arein the vapor phase when they contact the catalyst bed. Such reactionzones are known to persons of ordinary skill in the art of hydrocarbonprocessing, and are described in patents such as U.S. Pat. Nos.3,838,038; 4,040,794; 4,567,023; each of which is incorporated herein byreference. Catalyst is withdrawn from the bottom of the reaction zoneand may be transported to a regeneration zone.

In the case of a moving bed regeneration zone, a multi-step regenerationprocess is typically used to recondition the catalyst to restore itsfull reaction-promoting ability. Catalyst flows by gravity through thevarious regeneration steps and then is withdrawn from the regenerationzone and furnished to the reaction zone. Multi-step regeneration zonesare know to persons of ordinary skill in the art of hydrocarbon catalystregeneration, and are described in patents such as U.S. Pat. Nos.3,652,231; 5,001,095; 5,053,371; and 5,227,566; each of which isincorporated herein by reference.

Movement of catalyst through the reaction or regeneration zone ispreferably continuous though, in practice, it may be semi-continuous. Bysemi-continuous movement is meant the repeated transfer of relativelysmall amounts of catalyst at closely spaced points in time. For example,one batch per minute may be withdrawn from the bottom of a zone andwithdrawal may take one-half minute; that is catalyst will flow forone-half minute. The duration of time between the end of the withdrawalof one batch and the start of the withdrawal of the next batch issufficiently short to prevent pinning of the catalyst in the zone. Ifthe inventory in the reaction or regeneration zone is large in relationto the batch size, the catalyst bed may be considered to be continuouslymoving. Suitable catalyst movement systems are known to persons ofordinary skill in the art of hydrocarbon processing, and are describedin patents such as U.S. Pat. Nos. 5,338,440 and 5,500,110, each of whichis incorporated herein by reference. Typically, the rate of catalystmovement through the catalyst beds may range from as little as 200pounds (90.7 kg) per hour to 4500 pounds (2041 kg) per hour, or more.

In the dehydrogenation of hydrocarbons, the feed stream to thedehydrogenation reaction zone usually consists of a combination of freshparaffins, a diluent material, and recycled unconverted hydrocarbons.Hydrocarbons which can be dehydrogenated include hydrocarbons havingfrom 2 to 30 or more carbon atoms per molecule, including normalparaffins, isoparaffins, alkylaromatics, naphthenes, and olefins, butthe hydrocarbons used are typically normal paraffins having from 3 to 22carbon atoms per molecule. The diluent material may be hydrogen, steam,methane, ethane, carbon dioxide, nitrogen, argon, or the like, or amixture thereof, but hydrogen is the most common diluent. When hydrogenis used as the diluent, it is used in amounts sufficient to ensure ahydrogen to hydrocarbon mole ratio of ordinarily from about 0.1:1 toabout 40:1, and more commonly from about 1:1 to about 10:1 in thereaction zone.

In dehydrogenation, the feed stream is heated and the bed of catalyst ismaintained at proper dehydrogenation conditions of temperature andpressure before the feed stream is passed to the reaction zone. Processdetails for combining and heating the feed stream are well known topersons having ordinary skill in the art. The reactants entering the bedare in vapor phase condition. The pressure in the dehydrogenation zoneis maintained as low as practicable, consistent with equipmentlimitations. The process may use one or more reaction subzones withheating means there between to ensure that the desired reactiontemperature can be maintained at the inlet to each reaction subzone.Additional diluent may be added into, or combined with the effluent of,any or all of the reaction subzones. Reaction conditions for thedehydrogenation of paraffins include a temperature of from about 752 toabout 1652° F. (400° C. to 900° C.), a pressure of from about 0.15 toabout 147 psi(g) (1 to 1013 kPa(g)), and a LHSV of from about 0.1 toabout 100 hr⁻¹. As used herein, the abbreviation ‘LHSV’ means liquidhourly space velocity, which is defined as the volumetric flow rate ofliquid per hour divided by the catalyst volume, where the liquid volumeand the catalyst volume are in the same volumetric units. Water or amaterial which decomposes at dehydrogenation conditions to form watersuch as an alcohol, aldehyde, ether or ketone, for example, may be addedto the dehydrogenation zone, either continuously or intermittently, inan amount to provide, calculated on the basis of equivalent water, about1 to about 20,000 wt-ppm of the hydrocarbon feed stream. About 1 toabout 10,000 wt-ppm of water addition gives best results whendehydrogenating paraffins having from 2 to 30 or more carbon atoms.

Dehydrogenation catalysts that meet commercial standards for activity,stability, selectivity, and physical strength (i.e., attritionresistance) are believed to be suitable for use in the subjectinvention. Dehydrogenation catalysts are described in U.S. Pat. Nos.3,274,287; 3,315,007; 3,315,008; 3,745,112; 4,430,517; and 6,177,381,each of which is incorporated by reference in its entirety. Typically,these catalysts are comprised of a platinum group component supported ona porous carrier material. The typical carrier material is a refractoryinorganic oxide such as gamma-alumina. Usually, dehydrogenationcatalysts contain on an elemental basis 0.01 to 2 wt-% platinum groupcomponent and about 0.1 to 5 wt-% of an alkali or alkaline earth metal.Often, there is present 0.05 to 1 wt-% platinum group component andabout 0.25 to 3.5 wt-% of the alkali or alkaline earth component. Theplatinum group component may be chosen from the group consisting ofplatinum, palladium, rhodium, ruthenium, osmium, and iridium, butplatinum is highly preferred. The alkali or alkaline earth component maybe selected from the group consisting of the alkali metals—cesium,rubidium, potassium, sodium, and lithium; and the alkaline earthmetals—calcium, strontium, barium, and magnesium. This component isusually either lithium or potassium. Another example of a suitabledehydrogenation catalyst is a catalyst which in addition to thepreviously described platinum and alkali or alkaline earth metalcomponents contains a tin component. This catalytic composite wouldcontain from about 0.1 to about 1 wt-% tin. Yet another catalyticcomposite which should be highly suited for use in the subject processcomprises an indium component in addition to the platinum, tin, andalkali or alkaline earth components. The indium component may be presenton an elemental basis equal to about 0.1 to about 1 wt-% of the finalcomposite. It is also known in the art that some catalytic composites ofthis nature may benefit from the presence of a small amount of a halogencomponent, with chlorine being the normally preferred halogen. Typicalhalogen concentrations in the final catalytic composite range from about0.1 to about 1.5 wt-%. A halogen component is not desired in allsituations. Layered composition comprising an inner core such as alphaalumina, and an outer layer, comprising an outer refractory inorganicoxide, bonded to the inner core are also suitable. These catalyticcomposites are known to those skilled in the art and are described inthe available references.

The effluent from the dehydrogenation reaction zone typically undergoestreatment for the separation of hydrogen by condensing the heavierhydrocarbons into a liquid-phase stream and the recovery of hydrogen andlight hydrocarbon gas. This gas can be recycled to the reaction zone andthereby supply the hydrogen diluent that is combined with the fresh andrecycled hydrocarbons. Generally, the hydrocarbon-rich liquid phase isfurther separated by means of either a suitable selective adsorbent, aselective solvent, or a selective reaction or reactions, or by means ofa suitable fractionation scheme. Unconverted dehydrogenatablehydrocarbons are recovered and may be recycled to the reaction zone.Products of the dehydrogenation reactions are recovered as finalproducts or as intermediate products in the preparation of othercompounds. Process details for the recovery and separation of productsfrom the effluent are well known to persons of ordinary skill in theart.

Catalytic reforming is used to improve the octane quality of hydrocarbonfeedstocks, the primary product of reforming being motor gasoline. Afeedstock is admixed with a recycle stream comprising hydrogen andcontacted with catalyst in a reaction zone. The usual feedstock forcatalytic reforming is a petroleum fraction known as naphtha and havingan initial boiling point of about 180° F. (82° C.) and an end boilingpoint of about 400° F. (204° C.). Reforming may be defined as the totaleffect produced by dehydrogenation of cyclohexanes anddehydroisomerization of alkylcyclopentanes to yield aromatics,dehydrogenation of paraffins to yield olefins, dehydrocyclization ofparaffins and olefins to yield aromatics, isomerization of n-paraffins,isomerization of alkylcycloparaffins to yield cyclohexanes,isomerization of substituted aromatics, and hydrocracking of paraffins.The catalytic reforming process is particularly applicable to thetreatment of straight run gasolines comprised of relatively largeconcentrations of naphthenic and substantially straight chain paraffinichydrocarbons, which are subject to aromatization through dehydrogenationand/or cyclization reactions. Further information on reforming processesmay be found in, for example, U.S. Pat. Nos. 4,119,526; 4,409,095; and4,440,626.

The reforming reaction zone may comprise one or more reactors withsuitable means between reactors if any to reheat each reactor's effluentand to assure that the desired temperature is maintained at the entranceto each reactor. The reforming reaction zone is operated at reformingconditions to achieve the desired reformate product quality. Reformingconditions include a range of pressures generally from atmosphericpressure (0 psi(g)) to 1000 psi(g) (0 to 6895 kPa(g)), and more commonlyfrom about 40 to about 200 psi(g) (276 to 1379 kPa(g)). The overall LHSVbased on the total catalyst volume in the reaction zone including allreactors is generally from 0.1 to 10 hr⁻¹, and more commonly from about1 to about 5 hr⁻¹. Hydrogen is supplied to provide an amount ofgenerally from about 1 to about 20, and more commonly less than about3.5, moles of hydrogen per mole of hydrocarbon feedstock entering thereaction zone. Water or a material which decomposes at reformingconditions to form water such as an alcohol may be added to the reactionzone, either continuously or intermittently, in an amount to provide,calculated on the basis of equivalent water, about 1 to about 1,000wt-ppm of the hydrocarbon feed stream. A hydrogen halide such ashydrogen chloride, or a material which decomposes at reformingconditions to form a hydrogen halide, such as an organic chloride, mayalso be added to the reaction zone in a similar manner, except in anamount to provide, calculated on the basis of equivalent halide, about0.5 to 1,000 wt-ppm of the hydrocarbon feed stream.

Catalytic reforming reactions are normally effected in the presence ofcatalyst particles comprised of one or more IUPAC Groups 8–10 (GroupVIIIA) noble metals (e.g., platinum, iridium, rhodium, palladium) and ahalogen combined with a porous carrier such as a refractory inorganicoxide. The halogen is normally chlorine. Alumina is a commonly usedcarrier. The preferred alumina materials are gamma, eta, and thetaalumina with gamma and eta alumina giving the best results. An importantproperty related to the performance of the catalyst is the surface areaof the carrier. Preferably, the carrier will have a surface area of from100 to about 500 m²/g. The particles are usually spheroidal and have adiameter of from about 1/16^(th) to about ⅛^(th) in (1.6–3.1 mm), thoughthey may be as large as ¼^(th) (6.3 mm). In a particular regenerator,however, it is desirable to use catalyst particles which fall in arelatively narrow size range. A preferred catalyst particle diameter is1/16^(th) in (1.6 mm). Examples of reforming catalysts that are suitablefor use in moving bed processes are described in U.S. Pat. No.5,858,205, which is incorporated herein by reference, and in thereferences cited therein.

Feed streams to dehydrocyclodimerization processes include saturated orunsaturated C₂–C₆ aliphatic hydrocarbons, such as isobutane, normalbutane, isobutene, normal butene, propane and propylene. Diluents suchas hydrogen or nitrogen may be in the feed stream.Dehydrocyclodimerization conditions for the dehydrocyclodimerization ofC₂–C₆ aliphatic hydrocarbons to aromatics include a temperature of fromabout 662 to about 1202° F. (350 to 650° C.), a pressure of from about 0to about 300 psi(g) (0 to 2068 kPa(g)), and a LHSV of from about 0.2 toabout 5 h⁻¹. Typical process conditions are a temperature in the rangefrom about 752° F. to about 1112° F. (400 to 600° C.), a pressure in orabout the range from about 0 to about 150 psi(g) (0 to 1034 kPa(g)), anda LHSV of between 0.5 to 3.0 hr⁻¹. Processes for thedehydrocyclodimerization of aliphatic hydrocarbons containing from 2 to6 carbon atoms per molecule to produce a high yield of aromatics andhydrogen are well known to persons having ordinary skill in the art.Such processes are described in U.S. Pat. Nos. 4,654,455 and 4,746,763,which are incorporated herein by reference.

Suitable dehydrocyclodimerization catalysts include those that contain aphosphorus modified alumina (aluminum phosphate) as a binder in additionto a zeolite and a gallium component. The zeolites which may be used areany of those which have a molar ratio of silicon per aluminum of greaterthan about 10 and a pore diameter of about 5 to 6 Angstroms. Specificexamples of zeolites which can be used are the well-known ZSM family ofzeolites, which includes ZSM-5, ZSM-8, ZSM-11, ZSM-12, and ZSM-35. Theamount of zeolite present in the catalyst is usually from about 30 toabout 90 weight percent of the catalyst. The phosphorous can beincorporated with the alumina in any acceptable manner known in the art,such as the gellation of a hydrosol of alumina which contains aphosphorus compound using the well-known oil-drop method. The relativeamount of aluminum and phosphorus expressed in molar ratios of aluminumper phosphorus ranges from about 1:1 to 1:100 on an elemental basis. Thegallium component is deposited onto the support in a manner whichresults in a uniform dispersion of the gallium, such as by impregnatingthe support with a salt of the gallium metal. The amount of galliumwhich is deposited onto the support varies from about 0.1 to about 5weight percent of the finished catalyst expressed as the metal. Furtherdetails of preparing and one method of preparing suitabledehydrocyclodimerization catalysts are described in U.S. Pat. No.4,692,717, the teachings of which are incorporated herein by reference.

It is common in the practice of dehydrogenation, reforming, anddehydrocyclodimerization reactions for the catalyst particles to becomedeactivated as a result of mechanisms such as the deposition of coke onthe particles. That is, after a period of time in use, the ability ofcatalyst particles to promote reactions decreases to the point that thecatalyst is no longer useful. The catalyst must be reconditioned, orregenerated, before it can be reused in the process. Thus, it is commonto remove catalyst from the bottom of a reactor in the reaction zone,regenerate it by conventional means known to the art, and then return itto the top of that reactor or another reactor in the reaction zone.

Three common examples of catalyst regeneration steps that can beperformed by circulating a recycle gas transversely through adown-flowing catalyst bed are combustion of coke deposits on thecatalyst, redispersion of metal on the catalyst, and reestablishing adesired halide (i.e., chloride) level on the catalyst. These arewell-known steps and are described in U.S. Pat. Nos. 5,151,392;5,457,077; and 6,153,091; each of which is incorporated herein byreference, and in the references cited therein. The following operatingconditions are typical for regenerating reforming catalyst, but are notmeant to exclude other catalysts or their regeneration conditions. Incoke combustion, the recycle gas stream typically contains a lowconcentration of oxygen (i.e., from about 0.5 to about 1.5 vol-%) passesthrough the catalyst. The coke is usually oxidized at temperaturesranging from about 900 to about 1000° F. (482 to 538° C.), buttemperatures in localized regions may reach about 1100° F. (593° C.) ormore. In addition to oxygen, the recycle gas also typically containswater and halogen-containing compounds, but the balance of the recyclegas is mostly nitrogen and carbon oxides. In metal redispersion, therecycle gas generally contains a higher concentration of oxygen (usuallyfrom about 2 to about 21 vol-%) and a lower concentration of water thanused for coke combustion. The recycle gas also generally contains eithermolecular chlorine or another chlorine-containing molecule that can beconverted in the regeneration zone to chlorine. The concentration ofmolecular chlorine that promotes rapid and complete redispersion of theplatinum metal depends on the amount of metal to be redispersed, but isusually on the order of from about 0.01 to about 0.2 mol-% of the gas inthe redispersion zone. The redispersion zone is maintained at conditionsthat favor an equilibrium shift towards molecular chlorine. The recyclegas contacts the catalyst at a temperature between about 800 and about1100° F. (427 to 593° C.) and temperatures within the redispersion zoneare usually in a range of from about 950 to about 1000° F. (510 to 538°C.). In rehaliding, the recycle gas must contain a halogen-containingmolecule that is suitable for adding halide to the catalyst, such ashydrogen chloride when the halide is chloride. The concentration ofhalogen-containing compounds in the recycle gas depends on many factors,including the hydroxyl content of the catalyst and the desired amount ofhalide to be added to the catalyst. The recycle gas may contain 1.0 to10 mol-% oxygen, but this concentration depends on many factorsincluding the composition of the catalyst and the concentration ofoxygen needed to decompose the halogen-containing molecule and/or todeposit halide (e.g., chloride) on the catalyst. Typically therehaliding zone will have a temperature of from about 400 to about 1000°F. (204 to 538° C.).

The following examples are illustrative of the invention and are onlyintended to illustrate the invention. It is not intended that theseexamples limit the scope of the invention as set forth in the claims.

EXAMPLES

In these examples, particulate material comprising spherical aluminaparticles having a diameter of 1/16 in (1.6 mm) was used. A model of thelongitudinal section of a vertically-oriented cylindrical vessel thatprovides for horizontal, radially-inward air flow andvertically-downward particle flow was used. The model contains a largerdiameter inlet screen and a smaller diameter outlet screen with a spacebetween the screens for a particle bed. The model has an air inlet fordelivering compressed ambient air to the inlet screen, an air outlet forwithdrawing air exiting the outlet screen, a particle inlet forsupplying particles to the top of the bed, and a particle outlet forwithdrawing particles from the bottom of the bed. The sizes andlocations of the particle inlets and outlets in relation to the particlebed were the same in Examples 1–13. The sizes and locations of theparticle inlets and outlets in relation to the particle bed were thesame in Examples 14–18. Within the model, the temperature was ambientand the pressure was less than 1 psi (6.9 kPa) above atmosphericpressure. All air velocities are superficial velocities and are computedat standard conditions of 60° F. (15.6° C.) and one atmosphere, and thesuperficial velocities are computed at the outlet screen cross-sectionalarea.

Example 1 (Comparative)

At the start of Example 1, the particle bed was full of particles, theparticle outlet was closed, and the particle bed had been stagnant forabout 2 days. Then, the air inlet was opened, and an air velocity of 5.1ft/s (1.6 m/s) through the bed was maintained. Then, the particle inletand outlet were opened to start the downward flow of particles. At acertain elevation from the bottom of the bed, downward particle velocitywas measured visually as a function of horizontal (transverse) distancefrom the outlet screen, at distances of 1.5%, 4.2%, and 8.3% of thedistance from the outlet screen to the inlet screen. The movement ofparticles is shown in Table 1. An “O” in Table 1 indicates a downwardparticle velocity of less than 0.2 mm/min, which was the lower limit ofmeasurable particle velocity using the unaided human eye. That is, forpurposes of these examples, particles having a downward velocity of lessthan 0.2 mm/min were deemed to be pinned. An “X” in Table 1 indicates adownward particle velocity of more than 0.2 mm/min. Referring to Table1, particles near to the outlet screen, that is within about 1.5% of thedistance from the outlet screen to the inlet screen, were pinned.Example 1 indicates that the stagnant bed pinning flow rate for the bedcorresponds to a velocity of less than 5.1 ft/s (1.6 m/s).

Example 2 (Comparative)

About 15 minutes after the start of Example 1 and after the particlevelocities were measured in Example 1, the air inlet was closed whilethe particle inlet and outlet were kept open so that particles floweddownward through the particle bed. The downward particle velocity wasmeasured at the same elevation and in the same manner as in Example 1,and the movement of particles is shown in Table 1, which shows that noneof the particles were pinned.

Example 3

About 15 minutes after the start of Example 2 and after the particlevelocities were measured in Example 2, the air inlet was re-opened whilethe particle inlet and outlet were kept open so that particles floweddownward through the particle bed. An air velocity of 5.1 ft/s (1.6 m/s)through the bed was maintained. The downward particle velocity wasmeasured at the same elevation and in the same manner as in Example 1,and the movement of particles is shown in Table 1, which shows that noneof the particles were pinned, even though the air velocity was the sameas that in Example 1.

Example 4

About 15 minutes after the start of Example 3 and after the particlevelocities were measured in Example 3, the air inlet was opened furtherwhile the particle inlet and outlet were kept open so that particlesflowed downward through the particle bed. An air velocity of 5.5 ft/s(1.7 m/s) through the bed was maintained. The downward particle velocitywas measured at the same elevation and in the same manner as in Example1, and the movement of particles is shown in Table 1, which shows thatnone of the particles were pinned, even though the air velocity was evenmore than that in Example 1.

Example 5

About 15 minutes after the start of Example 4 and after the particlevelocities were measured in Example 4, the air inlet was opened furtherwhile the particle inlet and outlet were kept open so that particlesflowed downward through the particle bed. An air velocity of 5.9 ft/s(1.8 m/s) through the bed was maintained. The downward particle velocitywas measured at the same elevation and in the same manner as in Example1, and the movement of particles is shown in Table 1, which shows thatparticles were pinned only to the same extent as in Example 1, eventhough the air velocity was 16% greater than that in Example 1.

Examples 4 and 5 show that, after the particle bed was moving and noneof the particles were pinned, pinning at the specified elevationoccurred when the velocity was increased up to the range of from about5.5 to about 5.9 ft/s (1.7 to 1.8 m/s) Thus, Examples 4 and 5 indicatethat the moving bed pinning flow rate for the bed corresponds to avelocity in the range of from about 5.5 to about 5.9 ft/s (1.7 to 1.8m/s). By contrast, Example 1 shows that, with the bed initiallystagnant, a velocity of about 5.1 ft/s (1.6 m/s) was sufficient to pinsome of the particles at the specified elevation. Thus, this inventionallows for the velocity to be increased by at least 0.4 ft/s (0.1 nm/s),or by at least 7.8%, without causing pinning to occur.

Example 6

About 15 minutes after the start of Example 5 and after the particlevelocities were measured in Example 5, the air inlet was opened furtherwhile the particle inlet and outlet were kept open so that particlesflowed downward through the particle bed. An air velocity of 6.5 ft/s(2.0 m/s) through the bed was maintained. The downward particle velocitywas measured at the same elevation and in the same manner as in Example1, and the movement of particles is shown in Table 1, which shows thatparticles were pinned to a greater extent than that in Example 5.

Example 7

About 15 minutes after the start of Example 6 and after the particlevelocities were measured in Example 6, the air inlet was closed slightlywhile the particle inlet and outlet were kept open so that particlesflowed downward through the particle bed. An air velocity of 5.6 ft/s(1.7 m/s) through the bed was maintained. The downward particle velocitywas measured at the same elevation and in the same manner as in Example1, and the movement of particles is shown in Table 1, which shows thatthe particles were pinned to a greater extent than in Example 4.

A comparison of Examples 4 and 7 shows the effect of the pinning of theparticles that occurred in Examples 5 and 6. Although both Examples 4and 7 were conducted at approximately the same air velocity, theparticles were pinned to a much greater extent in Example 7 than inExample 4. It is believed that the additional catalyst that was pinnedin Examples 5 and 6 was not unpinned by merely decreasing the airvelocity to approximately that of Example 4.

Example 8 (Comparative)

At the start of Example 8, the particle bed was full of particles, theparticle outlet was closed, and the particle bed had been stagnant for91 days. Then, the air inlet was opened, and an air flow through the bedwas initiated at a rate that would be sufficient to pin some of theparticles in the bed at a specified elevation within the bed when theparticle inlet and outlet were opened to start the downward flow ofparticles. Then, the particle inlet and outlet were opened to start thedownward flow of particles and the air flow rate was maintained constantfor about 15 minutes. Then the air flow rate was decreased to a lowerrate in order to unpin some of the pinned particles, and the new flowrate was maintained for about 15 minutes. The air flow rate wasdecreased step-wise six more times, each of which unpinned more of thepinned particles, with a 15-minute hold at each new flow rate prior tothe next step-down in flow rate. After the last step-down in air flowrate, none of the particles at the specified elevation were pinned. Theair velocities at the penultimate and final steps in flow rate wereabout 2.6 ft/s (0.79 m/s) and about 1.9 ft/s (0.58 m/s), respectively.

Example 9

After a 15-minute hold following the last step-down in air flow rate inExample 8, the air flow rate was increased to a velocity of about 3.8ft/s (1.2 m/s) and maintained at that flow rate for about 15 minutes.None of the particles at the specified elevation were pinned. Then, theair flow rate was increased to a velocity of about 4.4 ft/s (1.3 m/s)and maintained at that flow rate for about 15 minutes. Some of theparticles in the bed at the specified elevation pinned.

Example 9 shows that, after the particle bed was moving and none of theparticles were pinned, pinning at the specified elevation occurred whenthe velocity was increased up to the range of from about 3.8 ft/s (1.2m/s) to about 4.4 ft/s (1.3 m/s), which corresponds to the moving bedpinning flow rate. By contrast, Example 8 shows that, with the bedinitially stagnant, a velocity in the range of from about 1.9 ft/s (0.58m/s) to about 2.6 ft/s (0.79 m/s) was sufficient to pin some of theparticles at the specified elevation, and this range of velocitiescorresponds to the stagnant bed pinning flow rate. Examples 8 and 9 showthat the velocity can be increased by from about 1.2 ft/s (0.37 m/s) toabout 2.5 ft/s (0.76 m/s), or by from about 46% to about 132%, withoutcausing pinning to occur. The difference between the moving bed pinningflow rate determined in Example 9 and that determined in Examples 4 and5 is believed to be due in part to the longer stagnant period prior tothe start of Example 8 compared to that prior to the start of Example 1,and also to a greater extent of pinning in the bed in Example 8 comparedto that in Example 1. Thus, Examples 8 and 9 show that a moving bedpinning flow rate that is significantly greater than the stagnant bedpinning flow rate can be achieved, even though, unlike Example 2, theair flow rate in Example 8 was not decreased to zero. Because the airflow rate in Example 2 was decreased to zero, it follows that the airflow rate was decreased to a flow rate of 0% of the stagnant bed pinningflow rate in Example 2. In contrast, since the air flow rate in Example8 was decreased to a rate corresponding to a velocity of 1.9 ft/s (0.58m/s) and since the stagnant bed pinning flow rate in Example 8 wasdetermined to correspond to velocities in the range of from about 1.9ft/s (0.58 m/s) to about 2.6 ft/s (0.79 m/s), then the air flow rate inExample 8 was decreased to a flow rate of from about 73% to about 100%of the stagnant bed pinning flow rate. Therefore, even though the airflow rate in Example 8 was not decreased to zero or a low percentage ofthe stagnant bed pinning flow rate, nevertheless the moving bed pinningflow rate in Example 9 is significantly greater than the stagnant bedpinning flow rate in Example 8.

Example 10 (Comparative)

Example 10 was essentially a repeat of Example 8, except that theparticle bed had been reloaded about 1 day prior to the start of Example10, the air flow rate was decreased step-wise five more times instead ofsix more times, and the air velocities at the penultimate and finalsteps in flow rate were about 3.6 ft/s (1.1 m/s) and about 2.6 ft/s(0.79 m/s), respectively, instead of about 2.6 ft/s (0.79 m/s) and about1.9 ft/s (0.58 m/s), respectively.

A comparison of Examples 8 and 10 shows that as the time period that theparticle bed lay stagnant increased, the susceptibility of the bed topin also increased. In Example 10, the particle bed had been reloadedpreviously by only about 1 day, and at the specified elevation theparticles did not become entirely unpinned until the air velocity hadbeen decreased to the range of from about 2.6 ft/s (0.79 m/s) to about3.6 ft/s (1.1 m/s). By contrast, in Example 8, the particle bed had beenstagnant for 91 days, and at the specified elevation the particles didnot become entirely unpinned until the air velocity had been decreasedto the range of from about 1.9 ft/s (0.58 m/s) to about 2.6 ft/s (0.79m/s). Thus, as the time period during which the bed lay stagnantincreased, the amount of reduction in air velocity to unpin the bed atthe specified elevation also increased, which indicates that theparticles at the specified elevation of the bed were more susceptible topinning in Example 8 than in Example 10. This conclusion is confirmed byobservations of the beds in Examples 8 and 10 while the air velocitieswere being decreased step-wise. To illustrate, at an air velocity of3.4–3.6 ft/s (1.0–1.1 m/s) and at the specified elevation of the bed,particles within about 25% of the inter-screen distance from the outletscreen were pinned in Example 10 while particles within about 46% of theinter-screen distance were pinned in Example 8. Thus, at essentially thesame air velocity, the bed that had been stagnant for 91 days was about80% more pinned than the bed that had been reloaded only 1 daypreviously.

Example 11

Example 11 was similar to Example 9. After a 15 minute hold followingthe last step-down in air flow rate in Example 10, the air flow rate wasincreased and maintained at a new flow rate for about 15 minutes. Noneof the particles at the specified elevation were pinned. The air flowrate was increased again to a velocity of about 4.6 ft/s (1.4 m/s) andmaintained at that flow rate for 15 minutes. None of the particles atthe specified elevation were pinned. Then, the air flow rate wasincreased to a velocity of about 5.4 ft/s (1.6 m/s) and maintained atthat flow rate for about 15 minutes. Some of the particles in the bed atthe specified elevation pinned.

Example 11 shows that, after the particle bed was moving and none of theparticles were pinned, pinning at the specified elevation occurred whenthe velocity was increased up to the range of from about 4.6 ft/s (1.4m/s) to about 5.4 ft/s (1.6 m/s), which corresponds to the moving bedpinning flow rate. By contrast, Example 10 shows that, with the bedinitially stagnant, a velocity in the range of from about 2.6 ft/s (0.79m/s) to about 3.6 ft/s (1.1 m/s) was sufficient to pin some of theparticles at the specified elevation, and this range of velocitiescorresponds to the stagnant bed pinning flow rate. Examples 10 and 11thus show that the velocity can be increased by from about 1.0 ft/s(0.30 m/s) to about 2.8 ft/s (0.85 m/s), or by from about 28% to about108%, without causing pinning to occur. The difference between themoving bed pinning flow rate determined in Example 11 and thatdetermined in Example 9 is believed to be due in part to the shorterstagnant period prior to the start of Example 10 compared to that priorto the start of Example 8. Thus, Examples 10 and 11 show that a movingbed pinning flow rate that is significantly greater than the stagnantbed pinning flow rate can be achieved, even though, like in Example 8,the flow rate in Example 10 was not decreased to zero. Since in Example10 the air flow rate was decreased to a rate corresponding to a velocityof 2.6 ft/s (0.79 m/s) and the stagnant bed pinning flow rate wasdetermined to correspond to velocities in the range of from about 2.6ft/s (0.79 m/s) to about 3.6 ft/s (1.1 m/s), then the air flow rate inExample 10 was decreased to a flow rate of from about 72% to about 100%of the stagnant bed pinning flow rate. Therefore, despite the fact thatthe flow rate of air in Example 10 was not decreased to zero or to avery low rate, the moving bed pinning flow rate in Example 11 issignificantly greater than the stagnant bed pinning flow rate in Example10.

Example 12

At the start of Example 12, the particle bed was full of particles, theparticle outlet was closed, and the particle bed had been stagnant forabout 5 days. Then, the particle inlet and outlet were opened to startthe downward flow of particles. Then, the air inlet was opened, and anair velocity of 5.6 ft/s (1.7 m/s) through the bed was maintained. Thedownward particle velocity was measured at the same elevation and in thesame manner as in Example 1, and the movement of particles is shown inTable 1, which shows that none of the particles were pinned.

Example 12 illustrates starting the downward flow of particles prior tostarting the flow of air, in contrast to Example 1 where the downwardflow of particles was begun after the flow of air was started.

Example 13

About 15 minutes after the start of Example 12 and after the particlevelocities were measured in Example 12, the air inlet was opened furtherwhile the particle inlet and outlet were kept open so that particlesflowed downward through the particle bed. An air velocity of 6.5 ft/s(2.0 m/s) through the bed was maintained. The downward particle velocitywas measured at the same elevation and in the same manner as in Example1, and the movement of particles is shown in Table 1, which shows thatparticles were pinned to a greater extent than in Example 12.

Examples 12 and 13 indicate a range for the moving bed pinning flow rateof between about 5.6 ft/s (1.7 m/s) and 6.5 ft/s (2.0 m/s) for a bedthat had been stagnant for about 5 days. This range overlaps the rangefor the moving bed pinning flow rate that is indicated by Examples 4 and5 of between about 5.5 ft/s (1.7 m/s) and 5.9 ft/s (1.9 m/s). Therefore,a comparison of Examples 12 and 13 on the one hand with Examples 4 and 5on the other hand indicates that the same, or nearly the same, movingbed pinning flow rate is attained in a bed of non-pinned particleswhether the particles in the bed had previously been pinned and thenunpinned as in Examples 1 to 5 or merely stagnant for about 5 days as inExamples 12 and 13.

Example 14

At the start of Example 14, the particle bed was full of particles, theoutlet was closed, and the particle bed had been stagnant for about 30days. Then, the particle inlet and outlet were opened to start thedownward flow of particles. Then, the air inlet was opened, and an airvelocity of 4.1 ft/s (1.2 m/s) through the bed was maintained. Thedownward particle velocity profile was measured at the same elevationand in the same manner as in Example 1, and the movement of particles isshown in Table 1, which shows that none of the particles were pinned.Thus, the air velocity of 4.1 ft/s (1.2 m/s) corresponds to a flow rateof air that is less than the moving bed pinning flow rate.

Example 15

About 10 minutes after the start of Example 14 and after the particlevelocities were measured in Example 14, the particle outlet was closedfor 30 seconds so that particles stopped flowing downward through theparticle bed, and then the particle outlet was reopened. An air velocityof 4.1 ft/s (1.2 m/s) through the bed was maintained. About 3–5 minutesafter the particle outlet was re-opened, the downward particle velocityprofile was measured at the same elevation and in the same manner as inExample 1, and the movement of particles is shown in Table 1, whichshows that none of the particles were pinned.

Example 16

About 10 minutes after the start of Example 15 and after the particlevelocities were measured in Example 15, the particle outlet was closedfor 5 minutes so that particles stopped flowing downward through theparticle bed, and then the particle outlet was reopened. An air velocityof 4.1 ft/s (1.2 m/s) through the bed was maintained. About 3–5 minutesafter the particle outlet was re-opened, the downward particle velocityprofile was measured at the same elevation and in the same manner as inExample 1, and the movement of particles is shown in Table 1, whichshows that none of the particles were pinned.

Example 17

About 10 minutes after the start of Example 16 and after the particlevelocities were measured in Example 16, the particle outlet was closedfor 25 minutes so that particles stopped flowing downward through theparticle bed, and then the particle outlet was re-opened. An airvelocity of 4.1 ft/s (1.2 m/s) through the bed was maintained. About 3–5minutes after the particle outlet was re-opened, the downward particlevelocity profile was observed at the same elevation and in the samemanner as in Example 1, and the movement of particles is shown in Table1, which shows that none of the particles were pinned.

Example 18

About 5 minutes after the start of Example 17 and after the particlevelocities were observed in Example 17, the particle outlet was closedfor 30 minutes so that particles stopped flowing downward through theparticle bed, and then the particle outlet was re-opened. An airvelocity of 4.1 ft/s (1.2 m/s) through the bed was maintained. About 3–5minutes after the particle outlet was re-opened, the downward particlevelocity profile was measured at the same elevation and in the samemanner as in Example 1, and the movement of particles is shown in Table1, which shows that particles were pinned to a greater extent than inExample 17.

Examples 14–18 illustrate that the particle bed can be temporarilyprevented from moving for a period of time up to about between 25 and 30minutes without causing pinning to occur when particle movement isresumed.

TABLE 1 Example No. 1 2 3 4 5 6 7 12 13 14 15 16 17 18 Gas velocity,ft/s 5.1 0   5.1 5.5 5.9 6.5 5.6 5.6 6.5 4.1 4.1 4.1 4.1 4.1 (m/s) (1.6)(0)   (1.6) (1.7) (1.8) (2.0) (1.7) (1.7) (2.0) (1.2) (1.2) (1.2) (1.2)(1.2) Relative particle flow 1.8 1.0 1.8 2.0 2.1 2.3 2.0 2.0 2.3 1.5 1.51.5 1.5 1.5 rate through particle outlet Particles at 1.5% ◯ X X X ◯ ◯ ◯X ◯ X X X X ◯ distance from 4.2% X X X X X ◯ ◯ X ◯ X X X X X outletscreen 8.3% X X X X X ◯ ◯ X ◯ X X X X X (% of inter- screen distance)

1. A process for passing a fluid through a bed of particulate material,the process comprising: a) maintaining particulate material in avertically extended bed having a fluid inlet face, wherein the bed ismaintained between the fluid inlet face and an outlet partition having aperforated section extending over at least part of its length, whereinthe size of the perforations retains the particulate material whilepermitting fluid flow therethrough; b) passing an inlet fluid to thefluid inlet face and transversely through the bed; and c) withdrawingthe particulate material from the bottom of the bed while recoveringoutlet fluid from the perforated section of the outlet partition at anoperating flow rate that is not less than a stagnant bed pinning flowrate.
 2. The process of claim 1 further comprising, after steps (a)–(c)(d) recovering outlet fluid from the perforated section of the outletpartition at an operating flow rate that is less than the stagnant bedpinning flow rate, and (e) after step (d), recovering outlet fluid fromthe perforated section of the outlet partition at an operating flow ratethat is not less than the stagnant bed pinning flow rate.
 3. The processof claim 1 further comprising prior to step (c) recovering outlet fluidfrom the perforated section of the outlet partition at an operating flowrate that is less than the stagnant bed pinning flow rate.
 4. Theprocess of claim 1 further characterized in that the particulatematerial passes through the bed at a downward particulate velocity ofmore than 0.02 mm/min.
 5. The process of claim 1 further characterizedin that the fluid inlet face and the outlet partition are separated by adistance and that particulate material located within 10% of thedistance, as measured from the outlet partition, moves at a downwardparticulate velocity of more than 0.02 mm/min.
 6. The process of claim 1wherein the fluid inlet face is maintained by an inlet partition havinga perforated section extending over at least part of its length.
 7. Theprocess of claim 1 wherein the process is a hydrocarbon conversionprocess.
 8. The process of claim 1 wherein the particulate materialcomprises a catalyst and the process is a catalyst regeneration process.9. The process of claim 1 wherein the bed is annular.
 10. The process ofclaim 1 wherein the operating flow rate is from 110 to 160% of thestagnant bed pinning flow rate.
 11. The process of claim 1 wherein theparticulate material in the bed is prevented from being at rest for aperiod of time of more than 0.5 hr.
 12. The process of claim 1 furthercharacterized in that the particulate material in the bed is preventedfrom being at rest for a period of time, the bed has a solid fraction ofthe particulate material, and during the period of time the solidfraction increases by less than 0.04.
 13. The process of claim 1 whereinparticulate material having an average size of less than half of theaverage size of the particulate material in the bed comprises less than1 wt-% of the particulate material in the bed.